1. Field of the Invention
The present invention relates to an organic electrophotographic photoconductor, and more particularly to an electrophotographic photoconductor comprising one photoconductive layer (hereinafter referred to as a single-layered type photoconductor) which is used with a positive charging process.
2. Discussion of Background
An electrophotographic process is employed to form a visible image from a latent electrostatic image. Therefore, it is necessary that an electrophotographic photoconductor for use with the electrophotographic process acquire sufficient chargeability in the dark and the surface potential of the photoconductor quickly decay by the application of light thereto. The above-mentioned characteristics of the photoconductor required on the electrophotographic process depend on the physical values of a photoconductive material for use in the photoconductor, such as high resistivity in the dark, excellent quantum efficiency and high electric charge mobility.
A photoconductor comprising an inorganic compound such as selenium, selenium - tellurium alloy or selenium arsenide, which is considered to meet the above-mentioned physical values, is conventionally employed in various kinds of copiers and printers. The inorganic photoconductor has, however, the drawbacks that those inorganic compounds are toxic, they must be handled with the utmost care because they are used in the amorphous state, and the manufacturing cost of the photoconductor is high because it is required to work each of the above-mentioned inorganic materials into a thin layer with a thickness of several ten .mu.m by vacuum deposition. The inorganic photoconductors thus obtained do not satisfy all the requirements for the electrophotographic photoconductor.
To solve the above-mentioned problems, organic photoconductors (OPC) using organic materials have been actively developed and they are put to practical use. Most of the organic photoconductors in the practical use have a laminated structure, comprising a charge generation layer (CGL) capable of generating an electric charge and a charge transport layer (CTL) capable of transporting the electric charge, and are used with the negative charging process. Namely, the negatively chargeable laminated-type photoconductors comprising the CGL and the CTL are current in this field. The reasons for this are as follows:
(1) In a photoconductor comprising a single-layered photoconductive layer in which necessary components are merely dispersed, the fatigue in the chargeability, the photosensitivity and the electrostatic properties becomes apparent in the course of repeated operations and these characteristics deteriorate below the practical level. In the laminated-layered type photoconductor, on the other hand, the deterioration in the above-mentioned characteristics can be prevented to a great degree. Further, in the case of the laminated-layered photoconductor, sufficient mechanical durability can be imparted to the photoconductor by the provision of the CTL on the surface side of the photoconductor because the CTL has excellent mechanical strength and the thickness of the CTL can freely be controlled to some extent.
(2) An organic material with such a high electric charge mobility as to cope with the high-speed electrophotographic copying process is almost limited to a donor compound showing only the positive hole transporting characteristics. When the CTL comprising the above-mentioned donor compound is situated on the surface side of the photoconductor, the obtained photoconductor necessarily acquires a negative chargeability.
However, the above-mentioned function-separating laminated-type photoconductor with the negative chargeability has the following shortcomings:
(a) One of the shortcomings results from the negative chargeability of the photoconductor. The charging by use of corona discharge is so much reliable in the electrophotographic process that many copiers and printers employ the corona charging method. As is known, however, the corona discharge of negative polarity is unstable as compared with that of positive polarity. The scorotron is therefore used for negatively charging the photoconductor, which increases the running cost. Furthermore, generation of a large quantity of ozone accompanies the negative corona discharge. The copier or printer employing the negatively-chargeable photoconductor is equipped with an ozone filter to prevent the ozone from being discharged from the apparatus. This also causes the increase of the cost. The amount of ozone generated from the apparatus is originally small in the positive charging process.
In addition, the demand for a positively-chargeable photoconductor is increasing because when a two-component type developer prevailing in the field of electrophotography is used in combination with the positively-chargeable photoconductor, the images can be obtained steadily due to excellent environmental stability of the developer.
The negative chargeability of the above-mentioned function-separating laminated-type photoconductor restricts the electrophotographic processes. Such a photoconductor cannot flexibly cope with various kinds of electrophotographic development process. For example, a photoconductor which is positively and negatively chargeable can cope with both the positive and negative charging processes and achieve the composite electrophotographic development process using the positive and negative charging systems.
(b) The second shortcoming of the previously mentioned function-separating laminated-type photoconductor with the negative chargeability is derived from the photoconductive layer with a laminated structure. The photoconductive layer of an organic photoconductor can be prepared by solution coating, which can reduce the manufacturing cost as compared with the inorganic photoconductor which is prepared by vacuum deposition. To prepare the laminated-type organic photoconductor comprising the CGL and the CTL, however, an operation for solution coating is required at least twice. When an undercoat layer is provided between a support and a photoconductive layer to ensure the chargeability, it is necessary to repeat the operation for solution coating three times. The increase in the number of solution coating operations causes the rise of the manufacturing cost. In addition to the above, it is necessary to remarkably severely control the thickness of the CGL to such a degree that the thickness is less than 1 .mu.m for the purpose of maintaining the balance between the photosensitivity and the mechanical durability of the photoconductor and obtaining excellent images. Severe control of the CGL thickness also becomes a factor in the increase of the manufacturing cost of the photoconductor.
With the above-mentioned shortcomings of the negatively-chargeable laminated-type photoconductor taken into consideration, it is understood that a single-layered organic photoconductor with positive chargeability is desirable in the electrophotographic process. Furthermore, if such a positively-chargeable photoconductor can be used in the negative charging process as it is or by making slight modification, the degree of freedom in the operating conditions can be increased at a low cost.
However, there are few organic photoconductors which can satisfy the above-mentioned requirements. At the present stage, examples of the single-layered organic photoconductor are almost limited to the followings: (1) a single-layered photoconductive layer comprising a charge transfer complex of polyvinylcarbazole serving as an electron donative material and trinitrofluorenone serving as an electron acceptable material; (2) a single-layered photoconductive layer comprising a eutectic complex of thiapyrylium dye and polycarbonate; and (3) a single-layered photoconductive layer in which a perylene pigment and a hydrazone compound are dispersed in a binder resin.
The above-mentioned conventional single-layered organic photoconductors (1) and (2) have the shortcomings that the sensitivity is so low that these photoconductors cannot endure the repeated operations, and a lot of ozone is generated because these photoconductors are used with the negative charging in the electrophotographic process. The single-layered organic photoconductor (3) is not suitable for high-speed copying process because of low photosensitivity.
Even though all the necessary components contained in a photoconductive layer with a laminated structure of the commercially available laminated-type photoconductor are merely dispersed in a single-layered photoconductive layer, the charging potential of the obtained photoconductor is not sufficiently high and the photosensitivity is not satisfactory. Further, the problem that the charging potential and the photosensitivity are unstable in the repeated copying operations remains unsolved.
When the single-layered organic photoconductor comprises a single-layered photoconductive layer which comprises a binder resin and a charge generating pigment dispersed in the binder resin, the charge generating pigment also serves as a charge transporting material. However, any pigments cannot satisfy the charge transporting characteristics of both a positive hole and an electron, so that the photosensitivity of the obtained photoconductor is low and the chargeability deteriorates in the repeated operations because of accumulation of the electric charges. In addition, there is an induction period in which the surface potential of the photoconductor does not decrease immediately after the photoconductor is exposed to light images, and therefore the latitude in the potential capable of forming a latent electrostatic images is small.
To improve the mobility of the positive hole in the above-mentioned conventional single-layered photoconductor, there is proposed a single-layered photoconductor comprising a single-layered photoconductive layer which further comprises a positive hole transporting material. In such a single-layered photoconductor, however, the problems that the chargeability is low and the charging potential drastically decreases in the course of repeated operations cannot be solved at the present stage.